Originally Posted by thompsonks
I've tested v2 and v4. Both yield color vignetting and blur in the far corners until you stop down to a middle aperture. V2 might be a little better than v4, or that could have been sample variation (after all these years of use) or user/tester error.
40 Cron turned out to be the shortest focal length that you can count on to cover the frame well – clearly better than 35. (And Leica WATE and MATE both work well at all FL bcz they're retrofocus designs.)
With Kolari mod you can open up 1 or 1.5 stops farther. Sorry no examples bcz I now have the Kolari mod.
Based on my copies of the v4 Summicron 35 and the M-Rokkor 40mm I think I'd disagree. Both are definitely affected and blur towards the edges, but my 35mm Summicron is clearly better than the 40mm. The edges are still blurred wide open, but it lacks the spherical aberration of the 40 and field curvature isn't as pronounced. The rear element on the 35 is larger and I think that gives it an advantage over the small rear element of the 40 decreasing the average angle that light hits the sensor.
If you're looking for extreme corner to corner sharpness you won't get it with either lens, but that's never been what the 35 Cron is about anyway. It's tack sharp in the center from wide open and all the way across the short width of the the frame at about f/5.6. This lens just renders gorgeously with a sense of tactile three dimensionality that I don't see in my Zeiss 35mm f/1.4 ZA.
Wish I had some better shots to show, but these from thanksgiving will have to do. I'm pretty sure these are all the Cron, but a couple could be my 50 lux pre-asph or the M-Rokkor 40. Lack of metadata makes it hard to know for sure, haha. To be honest, all three are beautiful on the A7RII, with the 40mm a step below the others, in my opinion.